What is the etiology and what are the symptoms of Q fever?

Q Fever: Etiology and Clinical Manifestations

Q fever is a zoonotic disease caused by the intracellular bacterium Coxiella burnetii, transmitted primarily through inhalation of contaminated aerosols from infected animals, and typically presents as a nonspecific febrile illness, pneumonia, or hepatitis after a 2-3 week incubation period. 1

Etiology

Causative Organism:

• Coxiella burnetii is a strictly intracellular, gram-negative bacterium that lives within phagolysosomes of macrophages and monocytes 2, 3
• The organism is highly infectious, with as few as one organism capable of causing clinical infection 3

Transmission Routes:

• Primary route: Inhalation of aerosols from contaminated soil, dust, or animal waste 1
• Highest bacterial shedding occurs during parturition (birthing), with millions of organisms released per gram of placenta 2
• Contaminated birth products, feces, milk, and urine from infected animals serve as sources 4
• Airborne transmission can occur over considerable distances, and 60% of U.S. cases occur without reported direct livestock contact 1

Animal Reservoirs:

• Domesticated ruminants are the primary reservoirs: cattle, sheep, and goats 5
• Secondary reservoirs include dogs, cats, birds, rodents, and ticks 4
• Animals typically have latent infections with persistent environmental shedding 2

Clinical Symptoms

Acute Q Fever (Most Common Presentation)

Key Clinical Features:

• Incubation period: 2-3 weeks after exposure 1
• Asymptomatic rate: Up to 60% of infections result in asymptomatic seroconversion 1
• Symptomatic manifestations occur in approximately 50% of infected persons 1

Cardinal Symptoms in Adults:

• Fever: Median duration of 10 days untreated (range 5-57 days); 60% of patients >40 years have fever >14 days 1, 6
• Severe, debilitating headache: Often retroorbital with photophobia, may radiate to jaw and be misdiagnosed as migraine or dental infection 1, 6
• Fatigue and myalgia: Among the most frequently reported symptoms 1, 6
• Chills 1

Three Primary Clinical Manifestations:

1. Nonspecific febrile illness (most common) 1
2. Pneumonia: Nonproductive cough in 50% of cases; upper respiratory signs notably absent; accounts for 2.3% of hospitalized community-acquired pneumonia 1, 6
3. Hepatitis 1

Less Common Acute Manifestations:

• Pericarditis, myocarditis, aseptic meningitis, encephalitis, cholecystitis 1
• Maculopapular or purpuric rash in 5-21% of adult cases 1

Pediatric Presentation:

• Children have milder illness and are less likely to be symptomatic than adults 1, 6
• Gastrointestinal symptoms (diarrhea, vomiting, abdominal pain, anorexia) occur in 50-80% of pediatric cases 1
• Rash is more common in children (up to 50%) compared to adults 1, 6
• Illness is typically self-limited, though relapsing febrile illness lasting months can occur 1

Pregnant Women:

• May be less likely to have typical fever symptoms compared to other adults 1
• Risk of miscarriage, stillbirth, premature birth, intrauterine growth retardation, or low birthweight 1
• Adverse outcomes result from vasculitis or vascular thrombosis causing placental insufficiency 1

Chronic Q Fever (Rare but Serious)

Epidemiology and Timing:

• Occurs in <5% of patients with acute infections 1, 6
• Can manifest within months or several years after acute infection 1
• Can follow either symptomatic or asymptomatic initial infections 1

Primary Manifestation:

• Endocarditis in patients with preexisting valvular or vascular defects 1
• Routine blood cultures are negative 1
• Vegetative lesions visualized by echocardiography in only approximately 12% of patients 1
• Always fatal if untreated (compared to <2% mortality in acute Q fever) 1

Other Chronic Manifestations:

• Chronic hepatitis 3
• Chronic fatigue syndrome 3

Critical Epidemiologic Risk Factors

Occupational Exposures:

• Veterinarians, farmers, slaughterhouse workers, butchers, laboratory workers 1, 6
• However, 79% of U.S. cases occur in patients not in traditionally defined high-risk occupations 1

Geographic and Environmental Factors:

• Living in rural areas or within 10 miles of a farm housing livestock 1, 6
• Seasonal peak in spring, correlating with livestock birthing times 1
• Travel to endemic regions (Middle East, Netherlands outbreak areas) 1

Host Factors:

• Males have higher risk for symptomatic illness than females 1
• Highest age-related risk in persons aged 60-64 years 1
• Disease severity increases with age 1

Clinical Pitfall to Avoid

The absence of direct animal contact should NOT exclude Q fever from diagnostic consideration, as airborne transmission occurs and the majority of U.S. cases have no reported livestock contact 1, 6. Health-care providers must maintain clinical suspicion even without occupational risk or animal exposure history 1.